MobX : How to observe only selected part of the entire store?

Question

The variable Models will store an array of model objects. SelectedIndex will get changed by the UI through the action setSelectedIndex.

The Action addToResult should add the incoming data to the selected model and It should get reflected in the UI as well.

Here The problem is that, after executing the action addToResult, UI does not get updated.

 export default class Store {
  
  selectedIndex: number = -1

  @observable.deep activeModel: Model = undefined

      @action addToResult = (results: string[]) => {
    this.model[this.selectedIndex].results.push(results[0])
  }

  @action setSelectedIndex = (index: number) => {
    this.selectedIndex = index
    this.activeModel = {
      ...this.models[this.selectedIndex]
    }
  }

  models: Model[] = []

  constructor () {
    makeObservable(this)
  }

}